The present invention relates to an apparatus for surface-machining of workpieces made of hard-to-work materials and can be utilized in many branches of a country's economy for machining workpieces of steel, ceramics, hard alloys, quartz, ruby, silicon, saphire, glass, etc.
More particularly, the present invention relates to an apparatus for finishing workpieces with free abrasive on a surface-lapping machine.
According to the known apparatus for finishing workpieces the lapping tools are dressed kinematically directly in the process of lapping by the same workpieces being machined which is effected due to a special cyclic change in the value and direction of the rotation speeds of the operating elements of the finishing machine.
A disadvantage of this known apparatus consists in that during prolonged operation the lapping tools become mutually "worn-in" which increases the amount of required labour considerably and impairs the effectiveness of kinematic dressing. The "wearing-in" phenomenon with regard to the lapping tools is characterized by two features.
Firstly, the profile of the lapping tool acquires a definite shape of the worn working surface differing from the initial shape. In this case, when the workpiece is finished from two sides, the profiles of the upper and lower lapping tools become mutually matching, i.e. one of them becomes a mirror image of the other.
This disadvantage is caused by the error of the apparatus in which the velocities are selected with no regard to the redistribution of pressure over the surfaces of the gradually wearing lapping tools.
Secondly, the surface layer of the lapping tools acquires a maximum wear resistance due to the redistribution of internal stresses and the formation of a dynamic equilibrium (for the given wearing conditions at the given working duty) between the developing microscopic cracks, removed with the products of abrasion and responsible for the intensity of shaping the surfaces of the workpiece and lapping tools.
This results in that changes in the kinematic conditions in the prior art method make difficult the finishing of workpieces with simultaneous kinematic dressing of the lapping tools, the finishing time sharply increases so that the accuracy of finishing depends on the duration of the transitional dressing period.
One of the methods of eliminating the "wearing-in" phenomenon is the cyclic change in the external pressure in order to bring the system out of equilibrium.
In practice it is possible to stabilize the accuracy of the geometric shape of the machined surfaces of parts. Throughout the process of machining after achieving the required quality of the surface layer of the machined workpieces by two radically different methods, viz., by retaining the initial geometrical shape of the working surface of the lapping tool or by finding a combination of factors of the finishing process which at any given moment of time and a given condition of the working surface of the lapping tool would ensure minimum deviations from the required shape of the surface of the workpiece and the accuracy of obtained dimensions.
Also known in the prior art are machines for finishing the workpieces located between the lapping tools in the sockets of a holder mounted on an eccentric shaft which mounts a planet pinion meshing with the sun wheel of a planetary mechanism.
This layout of the machine does not permit changing the eccentricity of holder rotation and thus changing the trajectory of the workpiece moving over the lapping tools in the process of machining, i.e. it fails to ensure finishing with simultaneous kinematic dressing of the lapping tools.
The invention disclosed in the Author's Certificate No. 483229, B24b7/22 describes a surface-lapping machine designed for grinding and polishing flat surfaces which comprises a fixed lower lapping tool, a rotating upper lapping tool, a holder fixed on a gear linked kinematically with the sun wheel of a planetary mechanism, with a pinion cage and a carrier fixed on the pinion cage axle, said axle carrying a free-mounted idler gear located between the holder gear and the sun wheel while the holder gear is installed on an additional axle passing through the carrier which rests on the pinion cage (see Author's Certificate No. 483229, Cl. B24b7/22, USSR).
The disadvantages of this machine are as follows:
Impossibility of changing the ratio of rotation speeds of the pinion cage and holder gear on changes in eccentricity.
Impossibility of changing the eccentricity in the process of machining. The eccentricity can be changed only periodically, after stopping the machine.